[QGIS server] add legendUrl to layers delivered by getCapa Doc. substitutes pull request 767 and 774!

this commit adds legendUrl tags to the style section of a layer in the getCapa document.
It extends the layers metadata GUI in QGIs to make legendUrl customizable (linking to a
image url) providing the legend. If this is not set the Url is build from serviceUrl or main GetLegend
section of the getCapa doc.

this commit substitutes pull request 767 (#767) and adds a to each layer in STYLE to make QGIS mapserver work with clients reading GetLegend URLs directly from each LAYER. E.g. http://geoportal.bayern.de/bayernatlas
This format can also be seen on p. 42 of OGC 05-078r4 (http://www.opengeospatial.org/standards/sld SLD Version 1.1.0)